1. Field of the Invention
The present invention relates to a real time clock synchronizing system provided in network elements in synchronous transmission networks such as networks in accordance with the synchronous digital hierarchy (SDH) defined by CCITT recommendation G.707, 708, and 709, and the synchronous optical network (SONET) defined in ANSI T1.105. In the synchronous transmission networks, occurrences of alarm events are detected in individual network elements, and are reported to a centralized monitor apparatus, together with information on real times at which the alarm events have occurred. The centralized monitor apparatus collects the real time information concerning the occurrences of and recoveries from the alarm events. Therefore, each network element contains provision for supplying the real time information. It is desirable that all of the network elements supply the synchronized real times to the centralized monitor apparatus with high accuracy.
2. Description of the Related Art
When an alarm event has occurred in a network element in the synchronous transmission network, the network element generates information on the occurrence of the alarm event, and transmits the same to the centralized monitor apparatus. The information on the real time at which the alarm event has occurred, is included in the above information on the occurrence of the alarm event. The centralized monitor apparatus collects such information from network elements in the synchronous transmission network, and holds histories of the occurrences of alarm events in the respective network elements in the network.
In the conventional synchronous transmission networks, each network element contains a real time clock (RTC) circuit which generates a real time clock based on clock signals generated by a local oscillator contained in the real time clock circuit. Thus, the above information on the real time included in the above information on the occurrence of the alarm event, is generated based on the real time clock generated by the real time clock circuit.
However, in the conventional synchronous transmission networks having provision as above, considerable differences arise among real times generated in respective network elements due to temperature variation and voltage variation. Therefore, the centralized monitor apparatus cannot obtain accurate real time information from the network elements in the synchronous transmission network.